Many people think of the moon as a dead planetary body, both in terms
of volcanic and seismic activity. The Apollo missions in the 1970s, how-ever,
proved otherwise.

The
Apollo Passive Seismic Experiment (APSE), a network of four seismometers
that was completed in April 1972, recorded seismicity on the moon until
the network was shut down because of cost-cutting measures on Sept. 30,
1977. While the network was operational, it clearly demonstrated that
the moon is seismically active, albeit on a smaller scale than Earth.
The moon is less active because it is a one plate planet and
does not, therefore, contain plate boundaries, the sites of large seismic
events here on Earth. But plate boundaries aside, the moon exhibits seismic
activity on a similar scale to that of intraplate settings on Earth.

Astronaut Buzz Aldrin sets up
a seismometer on the moon in 1969. A network of seismometers operated
on the moon and recorded moonquakes there until the experiment
was shut down in 1977. Renewed interest in moon exploration is spurring
new lunar seismic efforts. Neil Armstrong took this picture, which shows
the Lunar Module and American flag in the background. Image is courtesy
of NSSDC, NASA.

With the advent of a new era of lunar exploration, the importance of
the Apollo seismic database has greatly increased, highlighting the many
unanswered questions about so-called moonquakes, the lunar equivalent
of earthquakes. Understanding moonquakes is especially important to any
future plans for a permanent lunar base, currently called for in President
Bushs space exploration plan. Having a long-term human presence
on Earths only moon requires a full evaluation of the seismic risk.

Moonquakes
Over the last three decades, many researchers, most notably perhaps Yosio
Nakamura of the University of Texas in Austin, have mined the Apollo seismic
database and identified four different types of lunar seismic events.
These are thermal moonquakes, deep moonquakes, meteoroid impacts and shallow
moonquakes.

Thermal moonquakes are small and were recorded in several locations by
the Apollo seismometers. These are the result of daily temperature changes
that produce stress and strain of lunar rocks in young craters, eventually
causing them to crack or slump, as evidenced by the repetition
of signals with nearly identical waveforms at the same time during each
lunar day. Seismic activity begins abruptly two days after lunar sunrise
and rapidly deteriorates after lunar sunset. Of the four types of moonquakes,
these events emit the lowest energy.

Deep moonquakes are also low in energy but are the most abundant type,
with more than 7,000 events recorded during the eight years of monitoring.
These small-magnitude events (generally less than magnitude 2) occur approximately
halfway between the surface and the center of the moon (700 to 1,200 kilometers
deep), at highly regular monthly intervals, indicating a strong association
with the tidal pull of Earth. Deep moonquakes originate from specific
locations, or nests, that produce a quake of unique waveform.
To date, researchers have identified 318 nests from the Apollo dataset.

Meteoroid impacts also register as seismic events. While most of the
energy of an impact is expended in excavating a crater, some is converted
to seismic energy. Between 1969 and 1977, the APSE network recorded more
than 1,700 events representing meteoroid masses between 0.1 and 1,000
kilograms. Events generated by smaller impacts were too numerous to be
counted.

The strongest type of lunar seismic event is the shallow moonquake. These
quakes were originally referred to as high frequency teleseismic
events, and seven of the 28 events that the APSE network recorded
were greater than magnitude 5. The depths at which these moonquakes originate
are shallow, likely between 50 and 200 kilometers, although
the exact depths are unknown because all recorded events were outside
the perimeter of the APSE network. These events are relatively rare and
not correlated with tides. While they appear to be associated with boundaries
between dissimilar surface features, the exact origin of these events
is still unclear. A recent development by Nakamura and colleagues suggests
shallow moonquakes may originate from interaction of the moon with nuggets
of high energy particles (strange quark matter) originating
from a fixed source outside the solar system.

The shallow moonquakes and those caused by meteoroid impacts pose potential
(and very real) hazards to establishing a long-term habitable facility
on the moon. Still, it is important to note that the seismicity generated
by meteoroids is unlikely to pose a significant threat to any structure,
unless the impactors mass is on the order of tons and the impact
is close to the site of the moon base. Shallow moonquakes, however, contain
more energy at high frequencies than quakes on Earth of similar magnitudes.

Evaluating risk
Of critical importance to a lunar base is the amount of ground motion
associated with shallow moonquakes. As already noted, seven of the 28
shallow seismic events recorded by the APSE network had magnitudes of
5 or more. On Earth, such quakes can cause structural damage around the
epicenter, often manifested by the propagation of cracks through and along
walls. In a lunar setting, a magnitude-5 or greater quake could breach
the seal of a moon base, resulting in a catastrophic loss of oxygen, which
would devastate the habitat.

Currently, we do not know the cause or exact locations (especially the
depths) of these strong, shallow moonquakes, but they are in many respects
comparable to infrequent, highly destructive intraplate earthquakes. As
a first order approach, researchers have modeled ground motion and acceleration
using equations derived for terrestrial quakes (there are no such equations
as yet for the lunar scenario). Applying such terrestrial models to the
lunar environment is, at present, the best estimate we can make. However,
there are distinct differences in terms of seismic wave transmission between
the moon and Earth that could render the above estimates totally inadequate.

The Apollo Passive Seismic Experiment
involved deployment of a network of seismometers (outlined in the large
triangle) on the nearside of the moon. The seismometer at Apollo 11s
landing site failed after 21 days. The Apollo 17 seismometer was intended
to determine the shallow structure of the site and was switched off once
the experiment was complete. Image is courtesy of NASA.

For example, researchers have observed the maximum signal from a shallow
moonquake to last up to 10 minutes with a slow tailing off that can continue
for hours in total duration, demonstrating that signals dampen less while
traveling through the moon than they do traveling through Earth. This
effect suggests the mechanical properties of lunar rocks are distinct.

Studies have demonstrated the dramatic damping effect that water has
on seismic energy. Thus, seismic energy is more efficiently propagated
through the moon, which is incredibly dry. Significantly, moonquakes tend
to produce seismic waves of higher frequency than earthquakes. This consideration
is important for wave transmission through the lunar interior as well
as interaction with the near surface.

The material, or regolith, on the lunar surface has formed
through micro- to macro-sized meteoroid impacts. This incredibly dry unconsolidated
material tends to scatter seismic waves. Modeling of this scattering using
frequency-dependent diffusion appears to allow a statistical approach
to quantifying the data recorded by the APSE network. Using this model,
earlier researchers highlighted the fact that there is very low attenuation
of waves in the surface zone.

Known limits
While the APSE network has defined two potential hazards for any permanent
habitation on the moon, accurate risk assessment is difficult to formulate
because of the limitations of the current lunar seismic database. By integrating
the existing seismic data with strategically acquired new data, researchers
can not only better understand potential hazards, but also can better
refine scientific models about the moons interior structure and
formation.

The small area covered by the APSE network has resulted in difficulty
in estimating epicenter locations for seismic events. While attempts have
been made to locate the origins of both deep and shallow moonquakes, exact
locations require knowledge of the composition of the lunar mantle. The
small areal extent of the APSE network has resulted in reduced resolution
of seismic information from deep in the moons interior, limiting
interpretation.

For example, seismic data have been interpreted to indicate the presence
of garnet in the lunar mantle. However, the same seismic data were also
interpreted to represent an increased proportion of magnesium-rich olivine,
representing conflicting views. The presence of garnet in the lunar mantle
is also supported by the trace element ratios of some lunar volcanic glasses.
Resolving these differences to evaluate the moons composition is
important to any seismic risk evaluation, as it affects how seismic waves
travel through the body. More seismic data are necessary to better understand
the composition.

Likewise, very little seismic information is available regarding the
nature of the lunar core, as the APSE network was located on the nearside
of the moon. A large meteoroid impact on the farside of the moon produced
a seismic wave that was significantly delayed in reaching one of the Apollo
seismic stations. This event suggested the presence of a molten core approximately
720 kilometers in diameter, but no additional data were collected to confirm
this.

Geochemical and magnetic data also suggest that the moon does indeed
have a small core with a diameter estimated to range from approximately
500 to more than 800 kilometers. However, the composition of this core
has been reported to be anything from iron to iron sulfide to ilmenite
(which also contains iron). These compositions have important implications
for the thermal history of the moon. For example, if the core is predominantly
iron, it would be solid, but if it is iron sulfide, it could still be
(partially) liquid. Appropriate seismic data could rigorously test the
nature and composition of the lunar core.

The location of the APSE also did not provide much information regarding
deep seismic activity on the lunar farside. Early research reported 109
distinct nests of deep moonquakes, but only one of these was located on
the farside. A reevaluation of the APSE database found two more indisputable
farside nests with several other nests having epicenter locations on the
nearside, but uncertainty remains. These farside nests seem to produce
seismic waves that stop while traveling through the moons interior,
suggesting there is a plastic, or partially molten, zone located
somewhere in the deep lunar interior. Two major questions remain: 1) Are
there other nests on the lunar farside that were not detected by the APSE?
2) What is the nature and extent of the purported plastic zone?

More seismic data will not only help resolve these questions about the
moons interior, but also could inform hypotheses about the moons
formation. The lunar magma ocean model has become the cornerstone of our
understanding of moon evolution. This model postulates that immediately
after formation, the moon underwent a global melting event that affected
approximately the outer 500 kilometers of the moon or even all of it.

The APSE data have been interpreted to indicate the presence of a seismic
discontinuity at approximately 400 to 600 kilometers beneath the equatorial
moon that could represent the base of the lunar magma ocean, but it is
unclear whether it extends around the whole moon. If it does, then there
is evidence for the global melting event, although it may indicate that
only the outer skin of the moon melted. If it does not, then
a rethinking of the magma ocean model is required. Again, more seismic
data are necessary.

The next generation
Predicting where shallow moonquakes will occur is of prime importance
for the next phase of lunar exploration. Current data make this difficult
because only 28 such events were recorded before the APSE was shut down.

The relatively small number of events and the error in locating epicenters
has limited the statistical significance of relating these shallow seismic
events to tectonic features or a possible extra-solar-system origin. Thus,
the causes and locations of shallow moonquakes are not known with any
high degree of confidence. However, the existing data indicate that one
magnitude-5 or greater event occurs each year. Because the shallow quakes
are potentially the most destructive, such knowledge must be obtained
in the near, if not immediate, future.

Thus, an urgent need exists for a long-lived global seismic network to
be established on the moon. Over the last three years, without the help
of astronauts, an international team has been investigating the intricacies
of remotely deploying the Lunar Seismic Network (the LuSeN mission).

The seismometer that was chosen for this investigation was the Centre
National dEtudes Spatiales very broadband seismometer
that was developed for deployment on Mars, and is more sensitive than
those used in the APSE. However, the outcomes of this investigation highlighted
major technology gaps in the remote deployment of seismic networks on
airless bodies. The major issues include developing long-lived power supplies
(with a six-year minimum lifespan) that can survive the long lunar night
and planning for a hard versus soft landing deployment of the sensors.

The seismometers that formed the APSE network were powered by RTGs (Radio-isotope
Thermoelectric Generators) with a design output of 73 watts. The LuSeN
team has estimated that the maximum power required for the very broadband
seismometer, integrated with computerized recording and transmission components,
is between 2 and 4 watts. It is unclear whether electrical power in this
range can be maintained for a minimum of six years using conventional
solar-battery technology. An Apollo-sized RTG for each seismometer package,
however, would be overkill.

What is required is the development of so-called mini-RPS
(Radioisotope Power System) units that supply between 0.1 and 10 watts
of power. While this concept has been explored by NASA, actual fabrication
and testing of such units is not occurring, nor is it planned to occur
in the near future. However, the concepts presented by researchers at
NASAs Jet Propulsion Laboratory in Pasadena, Calif., in 2004 are
tantalizing for the LuSeN mission and any mission that attempts to deploy
a global seismic network on an airless body. For example, the mass of
the RPS unit would be between 3 and 6 kilograms, resulting in a total
mass for each seismometer package of less than 8 kilograms  much
lighter than using conventional solar-battery-power supplies, which cause
the packages to weigh much more than 10 kilograms each.

In general, mass is a critical limiting factor. If the mission calls
for a hard landing, each seismometer package will need cushioning
material to protect the delicate instrumentation as it drops from a low-altitude
orbiter. But if a soft landing deployment is planned, each
package will need its own individual retro-rocket assembly
to slow its landing. Either option adds more mass to each package, and,
especially if conventional solar-battery-power technology is used, a network
cannot be established with a single launch. At least two launches will
be required to establish a minimum network of eight seismometers. The
mission will, therefore, be cost-prohibitive using the current mission
programs available through NASA.

Hopefully, the LuSeN team will continue to try to meet the goals laid
out in January 2004 by President Bush of living and working [on
the moon] for increasingly extended periods of time and ultimately
establishing an extended human presence on the moon. Obtaining a better
understanding of lunar seismic activity is crucial for achieving the presidents
vision.

At this time, it is suspected, but not known, that seismic events could
seriously compromise a permanent lunar habitat. To fully evaluate this
risk, a long-lived, global lunar seismic network needs to be established.
Now is the time to start developing the new power-generation systems so
that the lunar seismic network can be established to monitor and locate
seismic hazards before we humans can make the moon our home.

Unearthing
the moons birth

Impacts are the key players in the moons and Earths
early histories. A leading hypothesis for the moons formation
focuses on the idea of a Mars-sized planet striking Earth shortly
after the solar system formed, and then ejecting debris that ultimately
became the moon. The energy from the impact caused the forming moon
to melt, creating a lunar magma ocean, which may have encompassed
the entire moon.

Evidence supporting this giant-impact hypothesis from
early Apollo missions includes lunar rocks with an oxygen isotopic
composition nearly identical to Earths mantle. Such a composition
is consistent with the moon forming mostly from Earths mantle
and partly from the mantle of the impacting body.

The giant-impact hypothesis, however, has come under fire in recent
years. Orange pyroclastic glass returned from the Apollo 17 mission
 the last manned American mission to the moon  contains
a greater abundance of volatile gases than other volcanic products
on the moon. But the energy from the giant impact would have resulted
in the moons interior being more depleted in volatile elements.
The primordial imprint found in the glass would have been vastly
altered in a giant impact (and subsequent melting in the magma ocean),
necessitating an alternative explanation for how the moon formed.

Some researchers have suggested that instead of the giant impact,
Earth captured the moon from another orbit, and that all planets
initially formed with cool primordial cores. Subsequent impacts
could have then created sufficient energy to create the lunar magma
ocean.

Sorting through these hypotheses to understand how the moon formed
addresses just one of many scientific questions that further moon
exploration can answer. A global, long-lived, lunar seismic network
will tell scientists more about the composition of the moons
interior, including the size of the core and potentially the depth
of any magma ocean, and in turn, inform discussions about its origins.Back to top

Neal is an associate
professor at the University of Notre Dame in Indiana and has been conducting
lunar research for 20 years. He would like to thank Yosio Nakamura, whose
patience and encyclopedic knowledge of lunar seismicity greatly improved
this story.